The advantages of polarizing optical elements, and most particularly polarizing ophthalmic lenses, are well known. In particular, such elements may remove interference reflections, which come from plane or quasi-plane horizontal surfaces, from the field of vision. This type of lenses, for example, is obtained conventionally in the ophthalmic industry using a polarizing film. The most conventionally used material for manufacturing polarizing films is PVA (polyvinyl alcohol). For ophthalmic applications, the PVA film can be used alone or sandwiched between two protective films, for example between two polycarbonate films, so as to form a less fragile composite film.
The polarizing ophthalmic lens is then obtained following two conventional routes used in the industry:
Via overmolding, by precisely positioning the polarizing film in a mold before casting a thermosetting monomer. After curing, the film is incorporated permanently into the volume of the ophthalmic lens substrate. PVA films alone are used for this.
Via injection molding, by positioning the polarizing film at the surface of an insert before injection-molding of a thermoplastic polymer. Such a technique is possible only if the side of the polarizing film in contact with the injection-molded polymer is a compatible thermoplastic material, that is to say capable of melting with the injection-molded material to give a durable assembly. It is applied in particular to composite films that are stacks based on a PVA film positioned between two protective films, and which may be thermoformed before being positioned in the insert. Furthermore, the protective film that covers the polarizing film on the side opposite to the optical element made from injection-molded material must not have significant birefringence as, in this case, the polarizing effectiveness of the optical element that is finally obtained may be reduced. This limits the variety of polarizing film that can be used in this type of method.
In the two methods, overmolding and injection molding, heating of the polarizing film causes a change in this film. In particular, its color and its polarizing effectiveness are modified involuntarily and in a not very reproducible manner. Furthermore, it is difficult to prevent uncontrolled displacements of the polarizing film from occurring during the overmolding.
Finally, the overmolding method does not allow an optical element to be obtained that has an optimized reduced total thickness. This is particularly irksome for high-refractive-index materials.